With medical x-ray imaging, x-ray tubes with rotary anodes are used to generate x-ray radiation. A plate-shaped rotary anode is accelerated to a high rotational frequency and is rotated away from electrons generated in a cathode (e.g., a “focal spot” at the point of incidence). The more quickly the rotary anode moves below the electron beam, the higher the possible surface loading of the focal spot. In order to improve the image quality of the x-ray imaging, there is the possibility of increasing the pulsed power of the x-ray tube or reducing the focal spot. With modern rotary anode x-ray tubes, the rotary anode rotates at approximately 150 Hz to 200 Hz. Accordingly, a powerful anode drive is provided in order to accelerate the mass of the rotary anode of several kilograms.
FIG. 1 shows a cross-section through an x-ray emitter 1 with an x-ray tube 2 and a rotary anode 3. A stator 6 outside of the vacuum vessel of the x-ray tube 2 but inside of the housing 10 of the x-ray emitter 1 generates an electromagnetic alternating field. The rotor 5 is disposed on a shaft 12 of the rotary anode 3. The rotor is made to rotate by the alternating field of the stator 6. A cathode 4 (e.g., an incandescent cathode with a Wehnelt cylinder) generates an electron beam 11 that is accelerated to the rotary anode 3. At the point of incidence on the rotary anode 3, the electron beam 11 is braked, and as a result, x-ray radiation 9 is generated. The generated x-ray radiation 9 leaves the x-ray emitter 1 through a beam exit window 7 of the housing 10 in order to be shaped by a diaphragm 8, for example.
The stator 6 may be operated with alternating current and consumes some kW of electrical power. The strong electromagnetic alternating field of the stator 6 has a disruptive influence on the trajectory of the electrons of the electron beam 11, since the electrons are deflected by the alternating field such that the focusing on the rotary anode 3 is disturbed. This results in a movement to and from the focal spot in time with the frequency of the alternating field of the rotary anode drive. This modulation of the focus point is noticeable in an x-ray image generated by x-ray radiation from a shake of the image edges. This unwelcome effect is referred to among persons skilled in the art as “focus shake”.
There are a number of known possibilities of reducing the focus shake at least partially. For example, a Mu metal shielding may be applied between the stator and the rear of the rotary anode, for example. Mu metal is a magnetically-soft nickel-iron alloy and has the property of shielding electromagnetic fields. Since the focusing of the electrons on the rotary anode is realized by coils, the electron path and thus the focusing may also be influenced by changing the electromagnetic field. This method is very expensive.
Another possibility includes synchronizing the stator with the image frequency. If, for example, a recording takes place with 30 images per second, the stator may be supplied with eight times 30 Hz of alternating current (i.e., 240 Hz). As a result, the drive phase is the same as each recording, and a focus shake may be minimized. However, this solution produces interfering noise during operation of the x-ray emitter.
A further solution is specified in patent application DE 10 2012 204 841 A1. A rotary anode x-ray emitter having an x-ray tube for generating x-ray radiation is specified in order to generate x-ray radiation. The x-ray emitter includes a rotary anode having a rotary axis and arranged in a vacuum vessel, a rotor arranged on the axis of rotation, and a stator that is arranged partially outside of the vacuum vessel. The stator generates an electromagnetic alternating field for driving the rotor. The stator includes at least one stator coil for generating the electromagnetic alternating field. The x-ray emitter further includes a cathode for generating an electron beam accelerated to the rotary anode. At least one counter coil for compensating for the electromagnetic alternating field of the stator coil is arranged in the region of the cathode or of the electron beam.